Circuit board with self-locking terminals

ABSTRACT

The present invention provides an apertured circuit board having self-locking terminals which partially cover at least some of the apertures. The terminals, which are integral extensions of the circuit pathways, are adhesively anchored to the circuit board at the terminal peripheries and are covered in part by a dielectric overlay. The latter also provides environmental protection and electrical insulation to the circuit conductive pathways. A particular feature and advantage of the invention resides in the provision of an integral tie-down shoulder on each terminal for capturing by the dielectric overlay. The circuit board and integral sockets may be formed by photoimaging and chemical resist techniques.

This application is a division of our copending application Ser. No.808808, filed June 22, 1977, for Circuit Board with Self-LockingTerminals, now U.S. Pat. No. 4,107,836 issued Aug. 22, 1978.

This invention relates to electrical circuit assemblies and methods ofmanufacturing the same, and more particularly to improvements in circuitboards and to methods for manufacturing the same.

In general, a printed circuit board comprises an electrically insulatingbase having an individual or composite electrical conductive pathways onat least one surface thereof, and one or more apertures communicatingbetween opposite surfaces on the insulating base. At least some of theapertures are surrounded at least in part by extensions of theconductive pathways ("terminals") which provide connection pointsbetween the conductive pathways and electrical and electroniccomponents. Numerous systems are well known in the art for mountingelectrical and electronic components and connectors onto printed circuitboards. Typically, the components are provided with leads, in which casethe components can be mounted to the board with their leads extendinginto and through the board apertures. Permanent electrical andmechanical connection between the components and the circuit board isachieved by soldering the components leads to the terminals surroundingthe apertures. The component leads may be individually soldered to theboard, e.g. as by hand soldering. However, connecting each lead in suchmanner is a tedious process. Accordingly, the art has developed varioussystems by which a plurality of soldered connections to a circuit boardmay be accomplished in a single mass soldering operation e.g. employingpot soldering or wave soldering techniques.

Although soldered connections are considered to be highly effective toestablish reliable electrical and physical connections between componentleads and a circuit board, the soldering operation adds significantly tomanufacturing costs. Furthermore, it generally is necessary to cleansoldered assemblies following soldering, and such cleaning also may addappreciably to manufacturing costs. Moreover, the relatively hightemperatures typically required by mass soldering operations may damageheat sensitive components and/or may warp or delaminate the circuitboard. Still another disadvantage of soldering is that replacing afaulty component is relatively time consuming and difficult, requiringboth unsoldering and re-soldering operations. Moreover, such unsolderingand re-soldering operations may result in damage to the circuit board,adjacent components or other soldered connections on the board.

Solderless wrapping, crimping and socketing are known in the art andoffer alternative methods to soldering for mechanically and electricallyconnecting electrical and electronic components onto printed circuitboards. Such methods rely on metal deformation of the jumper wire, thecomponent lead or socket elements, or both, to form a metallurgicallysealed electrical interface. Typically such methods require specialequipment, are generally labor intensive, and also may have certainconnection spacing requirements which limits their applicati on in thecase of high density configurations. Moreover, the stability of theinter-connections formed by such methods depend in part upon theresidual elastic stress remaining in the deformed metal, and thus mayfail in the field due to stress relaxation. An additional requirement ofsuch methods is to provide sufficient degree of metal deformation tobreak-down any surface oxides and other surface contaminations that maybe present on the mating metallic members, i.e. so as to assure a truemetal-to-metal contact. As a practical matter a common practice in theart is to plate the mating members prior to assembly with anon-corrosive metal such as gold. Such plating requirements may addsignificantly to manufacturing costs. Moreover, while such prior artsolderless interconnections theoretically offer an advantage oversoldered connections of easy separability, in practice solderlessinterconnections may be subject to degradation due to metal loss fromone or both contacting members upon repeated mating and unmatinginterconnections. Thus, such prior art solderless interconnectionspotentially offer advantages over soldered interconnections only incertain application.

Still other methods of forming interconnections between printed circuitboards and electrical components are described in Swengel U.S. Pat. No.2,958,064 and Hotine et al U.S. Pat. No. 3,275,736.

Swengel proposes forming a laminated circuit board with integral socketsfor gripping electrical component leads or the like. According toSwengel, one or more layers or sheets of a resilient and rubbery-typematerial are bonded to a circuit board. The resilient layer includesapertures corresponding in location to the circuit board apertures. Theapertures in the resilient layer are of a diameter slightly less thanthat of the lead to be inserted therein, whereby the resilient layer maymechanically grip a lead loaded in its apertures. In order to assureelectrical continuity a circuit extension, preferably integral with thecircuit pathways, is provided projecting into the aperture. An obviousdisadvantage of the Swengel circuit board is the need for the resilientsheets which may add to the cost of manufacture and may alsosubstantially increase the weight of the board. Moreover, the Swengel'sresilient sheets prevent visual inspection of the circuit, e.g. todetect defects. Still another disadvantage of Swengel is the reliance onthe mechanical force of deformation of a rubbery-type material to lockthe component and board together. As is well known in the art suchforces may lessen in time due to physical effects such as creep orstress relaxation and thus result in circuit failure in the field.

Hotine et al disclose an electrical connecting unit which basicallycomprises a plurality of relatively thin spring finger-like membersdisposed so as to extend partly over the board apertures. According toHotine et al the finger-like members have pointed ends, and areconfigured so as to define an aperture with respect to one another whichaperture is slightly smaller in dimension than the cross-section of thecomponent leads to be inserted therein. According to Hotine et al thefinger-like members mechanically lock the component leads to the board;however, the required electrical connections to the circuit pathways aremade by spot welding the individual finger-like members to the leads.Hotine et al reports that such redundant multiple welds result inimproved reliability of the interconnections in the field. Obviously,the Hotine et al system is relatively costly. Moreover, the Hotine et alsystem is not believed to be practical in the case of high densityapplications due to the connection geometry, and the requirement forsufficient tie down area for adhesive or mechanical attachment of theindividual finger-like members to the board. Moreover, sufficientconnector spacing is required so as to permit access to the individualfinger-like members for welding.

It is thus a principal object of the present invention to provide anovel interconnection system for electrically and mechanically attachingelectrical and electronic components to a printed circuit board. Anotherobject of the invention is to provide a solderless interconnectionsystem which overcomes the aforesaid problems of the prior art. Yetother objects of the present invention are to provide novel printedcircuit boards having integral solderless self-locking terminals formating with component leads, and to methods for producing circuit boardsof the type above described.

Generally, in accordance with the present invention a circuit boardhaving apertures therethrough is provided. The board has self-lockingterminals which partially cover at least some of the apertures. Theterminals, which are integral extensions of the circuit pathways, areadhesively anchored to the circuit board at the terminal peripheries andby a dielectric overlay. The latter also provides environmentalprotection and electrical insulation to the circuit conductive pathways.A particular feature and advantage of the invention resides in theprovision of an unique tie-down shoulder on each terminal whichincreases substantially the anchoring force provided by the dielectricoverlay. The circuit board and integral sockets may be formed byphotoimaging and chemical resist techniques as will be described indetail following.

As used herein the term "printed circuit board" is intended to refer tocircuit boards formed by conventional photo-imaging and resisttechniques as well as by stenciling and the like. The particular circuitdesign is a matter of choice and will be determined by desiredelectrical and electronic considerations well known to the art and whichform no part per se of the instant invention. The terms "electrical andelectronic components " are intended to refer to both active and passiveelectronic components, lead and jumper wires and the like.

For a further understanding of the nature and objects of the presentinvention, reference should be had to the following detailed descriptiontaken in connection with the accompanying drawings wherein like numbersdepict like parts and:

FIG. 1 is a top plan view of one form of circuit board constructed inaccordance with the present invention;

FIG. 2 is a side elevational view, partly in section of the circuitboard of FIG. 1;

FIG. 3 is an enlarged perspective view, showing a portion of the circuitboard of FIG. 1;

FIG. 4 is an enlarged side elevational view of the same portion of FIG.3, and showing an exemplary connection to an electrical lead;

FIG. 5 is a side elevational view, diagramatically illustrating aprocess for producing the circuit board of FIG. 1;

FIGS. 6-11 are perspective views of a circuit board at various stages offormation in accordance with the process of FIG. 5; and

FIG. 12 is a side elevational view, partly in section of an alternativeconstruction of circuit board in accordance with the instant invention.

One embodiment of connection system in accordance with the presentinvention is shown in FIGS. 1-4 of the drawings. In the embodiment ofFIGS. 1-4 the circuit board comprises a rigid dielectric panel or sheet30 of conventional circuit board insulating material, e.g. 0.062 inchthick phenolic resin board. (For convenience of illustration only aportion of the circuit board is shown in the drawings). A plurality ofelectrically conductive metallic pathways 32, each comprising anelongate central portion 34 extending between terminal ends 36 and 38,respectively, are provided on at least one surface, e.g. surface 39 ofpanel 30. For convenience of illustration, circuit pathways are shown ononly one surface of panel 30. It will be understood however, that panel30 may carry circuit pathways on both outer surfaces thereof, and ifdesired, panel 30 may comprise a plurality of insulating panels andincluding one or more internally carried layers of circuit pathways. Oneor more apertures 40 are provided through panel 30 communicating betweenopposite surfaces on the panel. The conductive pathways have dimensions,and shapes corresponding to desired design criteria, e.g. currentcarrying capacity and card geometry.

As seen in the drawings terminal ends 36, 38 and integral extensions ofcentral portions 34 surround apertures 40 at least in part. Apertures 40typically are round holes. As will become clear from the descriptionfollowing the diameter of apertures 40 will depend in part on thediameter of the lead to be loaded therein. For example, for a typicallead of about 0.030 inch diameter aperture 40 should have a diameter ofabout 0.070 inches. As seen particularly in FIGS. 1 and 3 each terminalend (e.g. 36) comprises a raised generally annular metallic member andincluding a plurality, e.g. four spaced spring fingers 42 each disposedpointing generally inwardly from the annulus of aperture 40 for adistance, but stopping short of the center of aperture 40. As will alsobecome clear from the description following, it is preferred that theinnermost, i.e. central edges of fingers 42 are generally concave, e.g.at 44, so as to provide an extending mating surface with the convexsurface of component leads 45 to be loaded therein (see FIG. 4). Thus,edges 44 of fingers 42 together define a generally round aperture 47,which is slightly smaller in dimension than the size of a typicalcomponent lead. For example, for a typical component lead of diameter of0.030 inches, fingers 42 may be shaped and dimensioned so as to definegenerally annular interspace of about 0.020 inches.

An important feature and requirement of the present invention is tosecurely anchor terminal ends 36 and 38 to the insulating panel 30.

Terminal ends 36 and 38 are called upon to perform the fourfoldfunctions of (1) electrically connect the conductor central pathways 34and component leads 45, (2) mechanically hold and maintain thecomponents, by their leads, to the insulating panel, (3) permit assemblyof components by simple mechanical deflection, and (4) apply relativelyhigh pressure against the component leads throughout the design life ofthe assembly.

A deficiency of prior art self-locking terminal boards, e.g. as taughtby Swengel is the failure to provide sufficient mating or hold-down areabetween the circuit terminals and the substrate board for adequatelyadhesively anchoring the terminals to the board. Sufficient hold-downarea is required so as to prevent the terminals from delaminating fromthe board during lead insertion or removal and also to ensure a constantoperating force. As will be appreciated providing sufficient hold-downarea between the terminals and the surrounding board may present aparticularly acute problem in the case of high density applications. Thepresent inventions offers a solution to this problem by providingterminal ends 36 and 38 with outwardly depending integral shoulders orsteps 46 of reduced thickness as compared with the main body of theterminal ends. For example, assuming the main body of the terminal endsincluding finger areas 42 have a thickness of about 0.01 inch, shoulders46 may have a thickness in the range of about 0.001 to 0.005 inches,preferably about 0.003 inches. Shaping the terminal ends in such mannermakes it possible to capture the reduced thickness shoulders 46 with anadhesive coated dielectric film overlay 48. Thus, as seen in FIGS. 2 and4 the terminal ends, i.e. the flat bottom surface of terminal end 36 andits associated conductor pathway 34 are bonded to the top surface ofrigid panel 30 by an adhesive 50. Film 48 is also bonded to the topsurface of panel 30 (except where presented by terminal ends 36 and 38and the conductor pathways 34). Film 48 also extends over terminalshoulders 46 whereby to capture and cover the shoulders 46 at least inpart. As seen in FIG. 3 film 48 preferably extends to the main, e.g.raised areas of terminal ends 36 and 38; however film 48 should notcover entirely terminal ends 36 and 38. Film 48 preferably is formed ofan electrically insulating polymeric film material such as a polyester,polypropylene, polyimide, cellulose triacetate, polyethyleneterephthalate or other readily available film. The thickness of film 48is not critical to the invention; however, film 48 would be relativelythin and should possess sufficient flexibility whereby the film can beshaped in close contact to the conductor pathways 34 and the shoulderareas 46 of terminal ends 36 and 38 with minimum film distortion. Film48 is bonded to the circuit rigid panel 30, terminal ends 36 and 38,terminal shoulders 46 and the conductor pathways 34 by suitable adhesivemeans such as a modified epoxy, or acrylic based adhesive.

FIGS. 5-11 illustrate one method of forming a printed circuit board inaccordance with the present invention.

An electrically conductive, resiliently flexible metallic sheet 60preferably of a thickness substantially equal to that desired for theterminal ends 36 and 38 raised areas and fingers 42 is provided. In theillustrated case metallic sheet 60 comprises 0.01 inch thick phosphorbronze ST. One skilled in the art will recognize however that otherelectrically conductive, resiliently flexible metallic materials may beemployed. The top and bottom surfaces 62 and 64 of the metallic sheet 60are then cleaned employing conventional techniques, and top and bottomsurfaces 62 and 64 are then coated at a coating station 66 (FIG. 5) withconventional acid resist materials layers 68 and 70, respectively. Thenone side of metallic sheet 60 (e.g. top side 62 and resist layer 68) isexposed, at an imaging station 72 to a negative art work image of thedesired raised areas of terminal ends 36 and 38 and fingers 42.Simultaneously bottom resist layer 70 is entirely exposed to light atimaging station 72. Those areas of resist coating 68 and 70 exposed tolight are altered to a lower molecular weight polymer. The sheet is thenimmersed in a preferential solvent and developed at a treating station74, with the result that the exposed bottom resist layer 70 and theexposed portions of resist layer 68 remain intact while the unexposedareas of coating 68 are dissolved leaving resist layer 68 in a positiveimage of the raised areas of terminal ends 36 and 38 and fingers 42.

The next step involves chemically milling the exposed metallic areas ofmetallic sheet 60 by contacting the sheet 60 with an acid etchingsolution at an etching station 78. Etching is controlled so as to removemetal to a depth which substantially equals that desired for the circuitconductor areas and terminal shoulders of the finished circuit borad.For example, if 0.003 inch thick circuit conductor pathways and terminalshoulders are desired, etching should be controlled to a depth of about0.007 inches. Spray etching has been found to be especially suitable forobtaining precision control of the etching step.

Thereafter, the etched sheet is treated in a stripping station 80wherein the acid resist remaining on the sheet is removed from bothsides of the sheet. A metallic sheet having a contoured surface 62 withraised areas 84 as shown in FIG. 7 results.

A feature and advantage of the present invention is the ability toeasily emboss dielectric overlay film 48 to the circuit board. As willbecome clear from the description following film 48 is called upon toperform the two-fold functions of (1) mechanically anchor the terminalsto the substrate panel and (2) electrically insulate the circuitpathways. Thus, in accordance with a preferred embodiment of theinvention the dielectric overlay film 48 is bonded to the FIG. 7structure. Thus, the next step in the process of the present inventionis to partially cover all but the raised areas 84 on the contouredsurface 62 of sheet 60 with a dielectric overlay in the form of a thin,flexible electrically insulating film such as one mil (0.001 inch)polyimide film 48. As shown in FIG. 8, film 48 is pre-punched so as tocover substantially the entire surface 62 of metallic sheet 60 otherthan raised areas 84. Film 48 is applied to sheet 50 at a coveringstation 86 (FIG. 5), and the film is bonded to the metallic sheet bymeans of a suitable adhesive, for example a thermoplastic adhesive suchas a modified acrylic (available from Dupont Company). One skilled inthe art will recognize an advantage of applying film 48 at this stage isthat the film will not be distorted by the circuit pathways (which arestill undefined). Moreover, the possibility of voids and pressure gapsis also reduced. Furthermore, since areas 84 are raised, the possibilityof adhesive flow onto the terminals (also not fully defined) isrelatively remote. The resulting structure appears substantially asshown in FIG. 8.

Another feature and advantage of the present invention is the ability toeasily and economically plate only the terminal areas of the circuitboard. Thus, in accordance with a preferred embodiment of the invention,the FIG. 8 structure is then returned to coating station 66 where thesheet lower surface 64 is again coated with layer 88 of a conventionalresist material. Then using one or more of the raised terminal areas 84to insure front-to-back image registration, resist layer 88 is exposedto a negative art work pattern which defines the actual terminalaperture 47, shoulder 46 and conductor 32 at imaging station 72. Thesheet is then treated in treating station 74 with the result thatexposed areas of the resist layer 88 remain intact while the unexposedareas are dissolved away as before. The resulting structure appearssubstantially as shown in FIG. 9. It should be noted that at this stagein the process metallic sheet 60, while contoured, still is continuous.

The next step involves plating the exposed metallic areas of sheet 60 bya conventional plating technique, e.g. by electrodeposition in knownmanner of a precious or semi-precious material such as gold at a platingstation 87. Inasmuch as the metallic sheet 60 is still continuous atthis stage of the processing, assuring electrical continuity for platingpurposes to each terminal end is assured. Film layer 48 effectivelymasks the entire top surface 62 of sheet 60 other than areas essentiallycorresponding to the raised areas of terminals 36 and 38 and fingers 42,while resist layer 88 effectively masks the entire bottom surface 64 ofsheet 60 except areas corresponding to the bottom surfaces of terminalends 36 and 38, fingers 42 and the conductors 32. As a result depositionof the precious metal is restricted essentially to those areas of sheet60 which ultimately will become parts of the terminals, i.e. platedareas 92 and 94. One skilled in the art will recognize an advantage ofthe present invention in that plating the terminals thus is anespecially simple procedure, and which may result in a relatively lowconsumption of precious metal. Still another advantage of the inventionis that the inner edges 44, i.e. the actual contact areas of theterminals, are also plated.

Following plating the resist layer 88 is then stripped from surface 64at stripping station 80. The resulting structure appears substantiallyas shown in FIG. 10.

Thereafter, using the plates areas 92 and 94 as a resist, the metallicstructure is returned to etching station 78 so as to complete thedefinition of the terminals, terminal fingers 42 and conductor 32. Thestructure which results is shown in FIG. 11.

At this point, the FIG. 11 structure is mounted at a mounting andlaminating station 98 onto a rigid panel or sheet 30 such as a 0.062 milphenolic resin board. The latter is predrilled with a plurality ofapertures 40 which correspond substantially to the placement on the FIG.11 structure of terminal ends 36 and 38. The panel 30 and FIG. 11structures are then laminated together in known manner, e.g. employing athermosetting adhesive. The resulting structure is a printed circuitboard of the type shown in FIGS. 1-4.

One skilled in the art will recognize that the mechanical locking forceof the terminal fingers can be varied to meet specific designrequirements, for example by employing different metallic substratematerial, changing the thickness of the metallic substrate material,adjusting the size or shape of apertures 40 or the size or shape offingers 42, or by a combination of one or more of the foregoing. Forexample, if a relatively large spring force is desired, the metallicsubstrate starting material (sheet 60) may be spring steel. In such caseit may be desired to plate-up the terminal contact areas (station 87)with a preferred electrically conductive material such as copper, orcopper followed by tin. Processing is otherwise as before. Moreover, ifdesired, the foregoing process can be modified so as to provide acircuit board having flexible conductor pattern area 100, e.g. byproviding rigid panels 30A, 30B adjacent and supporting only terminalends 36 and 38. Obviously, this will leave portions of the conductors 32exposed; however, if desired conductors 32 can be covered by a thin,flexible dielectric film 102 adhesively secured to the conductors 32,rigid panels 30A, 30B and dielectric overlay film 48 (FIG. 12).Moreover, the foregoing process of the present invention may also beadopted to produce two-sided or multi-layer circuit boards. Still otherchanges will be obvious to one skilled in the art.

What is claimed is:
 1. A circuit board assembly of the type adapted forsolderless attachment thereto of components by their leads to saidboard, and comprising in combination:(A) rigid, dielectric panel havingat least one aperture formed between opposite surfaces thereof; (B) atleast one electrically conductive circuit formed of a resilientlyflexible metal, rigidly affixed at least in part to a surface of saiddielectric panel, said electrically conductive circuit having (i) atleast one integral termination which essentially surrounds said at leastone aperture in said panel, said termination including (ii) a pluralityof spaced fingers (a) disposed inwardly for a distance from edge areasof said aperture, and (b) adapted for locking mating with said componentleads; and (C) an apertured, flexible, dielectric material fixedlyattached at least in part to said surface so as also to cover said atleast one conductive circuit at least in part, with its aperturespositioned over said apertures in said rigid, dielectric panel; saidintegral termination, and said fingers having areas of cross-sectionalthickness greater than the cross-sectional thickness of saidelectrically conductive circuit, said termination including a peripheralshoulder area (i) of reduced cross-section thickness and (ii) a planarea greater than said apertures in (a) said rigid, dielectric panel,and (b) said flexible, dielectric material, said peripheral shoulderbeing fixedly attached to said dielectric panel at least in part; andsaid flexible dielectric material overlaying and fixedly attached atleast in part to said termination peripheral shoulder area.
 2. A circuitboard according to claim 1 wherein said resiliently flexible metalcomprises a phosphor bronze.
 3. A circuit board according to claim 1wherein said spaced fingers and said conductive circuit are plated atleast in part with an electrically conductive metal.
 4. A circuit boardaccordiang to claim 1 wherein said resiliently flexible metal comprisesa spring steel.
 5. A circuit board according to claim 4 wherein saidspaced fingers and said conductive circuit are coated with anelectrically conductive metal.
 6. A circuit board according to claim 1wherein said flexible, dielectric material comprises a polymeric filmmaterial.
 7. A circuit board according to claim 1 wherein said fingersterminate with inwardly facing generally concave edges.
 8. A circuitboard according to claim 1 wherein said termination peripheral shoulderareas have a cross-sectional thickness in the range of 0.001 to 0.005inches, and said fingers and other areas of said termination have across-sectional thickness of about 0.01 inch.
 9. A circuit boardaccording to claim 8 wherein said termination peripheral shoulder areashave a cross-sectional thickness of about 0.003 inches.
 10. A circuitboard according to claim 1, and further including at least oneelectrical or electronic component electrically and mechanicallyconnected to said board with its lead(s) extending through and locked bysaid fingers in said at least one termination.
 11. A circuit boardaccording to claim 1 and including two rigid, dielectric panels havingelectrically conductive circuits formed of resiliently flexible metalextending between said two rigid, dielectric panels, said extendingcircuits being (i) integral with electrically conductive circuits onsaid rigid, dielectric panels, and (ii) covered at least in part by saidflexible, dielectric material.
 12. A circuit board according to claim 11and further including at least one electrical or electronic componentelectrically and mechanically connected to said board with its lead(s)extending through and locked by said fingers in said at least onetermination.